In butt welding two workpieces, such as steel plates, which are separated by a gap of varying width, the energy beam must be oscillated transversely to the longitudinal direction of the gap to be closed by the weld seam, and the width of oscillation must be at least equal, and preferably somewhat larger than, the maximum width of the gap to produce an acceptable weld seam of substantially constant width. Furthermore, additional or filler material must be supplied to the zone of welding to make up the deficiency in material corresponding to the gap, the supply of additional or filler material being generally controlled in dependence upon the width of the gap, see e.g. German Auslegeschrift No. 1,270,708.
Under the conditions outlined above, the amount of energy required for producing a weld seam of predetermined width should be substantially constant while the local energy demand needed for melting the respective portions of the workpieces to be joined, and for melting an additional material, varies as a function of the width of the gap. Thus, at a location where the width of the gap is relatively small and the feeding rate of the additional material is correspondingly low, the energy of the energy beam is consumed mainly in melting the marginal portions of the workpieces which form the gap, while in the case of a wide gap and a correspondingly high feeding rate of the additional material a large portion of the energy is used for melting the additional material.
It is known from U.S. Pat No. 3,131,289 to vary the wave-shape of the periodical signal which controls the transverse oscillation of an electron beam across a seam to be welded. This, however, varies the energy distribution across the seam rather than along the seam, because the frequency of transverse oscillation is maintained constant.
Further, U.S. Pat. No. 3,417,223 discloses an electron beam welding method wherein additional material is supplied to the welding site. In some embodiments of this known method, the relative amount of energy supplied by the beam to the workpiece material and the additional material, respectively, is controlled by deflecting the beam such that, in a first position, it strikes mainly the workpiece material, and in a second position it strikes mainly the additional material. However, the known method does not provide for measuring the width of a gap between two workpiece parts to be welded together and no control of the beam deflection in dependence of such width.